Field of the Invention
The present invention relates to a method and apparatus for reforming hydrocarbon fuel into a higher calorific fuel, and in particular relates to a method and apparatus for reforming fuel that does not require any of the catalysts commonly used in reforming hydrocarbon fuel.
Description of the Prior Art
Many processes, especially in industrial applications, produce large amounts of excess heat—i.e., heat beyond what can be efficiently used in the process. Waste heat recovery methods attempt to extract and utilize some of the energy from the excess heat that otherwise would be wasted. Typical methods of recovering heat in industrial applications include direct heat recovery to the process itself, recuperators, regenerators, and waste heat boilers.
One particular method of waste heat recovery is based on fuel thermochemical recuperation (TCR). A TCR system can include one or more recuperative reformers, one or more air recuperators, a steam generator, and other necessary components. TCR recovers sensible heat in flue gas (i.e., exhaust gas) from a thermal process (e.g., combustion in a furnace, engine, etc.) and uses that heat to endothermically transform a hydrocarbon fuel source (for example, a fossil fuel such as petroleum, natural gas, or landfill gas) into a reformed fuel with a higher calorific heat content. In particular, the reforming process uses hot flue gas components (such as H2O and CO2), steam, and/or CO2 (landfill gas) to convert the fuel into a combustible mixture of hydrogen (H2), carbon monoxide (CO), and unreformed hydrocarbons (CnHm).
The most studied and widespread reforming process is natural gas (methane) reforming with steam, known as a steam methane reforming (SMR). The SMR process is the most common method of hydrogen production. This process is realized by two main reactions: CH4+H2O→CO+3H2 and CO+H2O→CO2H2. The first reaction is strongly endothermic and usually realized at high temperatures (1380° F.-1470° F.) over a nickel catalyst. The second reaction, known as a water gas shift reaction, is mildly exothermic and usually realized at lower temperatures (370° F.-660° F.) over a nickel catalyst.
Natural gas reforming with flue gas is realized by the same two reactions and one additional endothermic reaction of methane with carbon dioxide: CH4+CO2→2CO+2H2. So in the TCR process, steam (H2O) and carbon dioxide (CO2) are reacting with fuel to produce reformed fuel with higher calorific value. In contrast to the SMR process, hydrogen production is not the only purpose of the TCR process. In the TCR process it is usually more important to increase calorific value of the fuel rather than produce hydrogen. Because of that, the exothermic water gas shift reaction is optional for the TCR process and can be eliminated.
Another possible reaction of the TCR and SMR processes is direct cracking of the hydrocarbon fuel. Cracking produces hydrogen and solid carbon. If the reforming process is conducted over a catalyst, the filamentous carbon eventually deactivates the catalyst. While catalysts are used in the SMR process for hydrogen production and cracking is undesirable, non-catalytic recuperative reforming would be very attractive for use in the TCR process when solid carbon can be utilized as a combustible together with the reformed fuel.
The calorific content of the fuel can be increased significantly. For example, if the original fuel source is natural gas (where methane is the main component), the calorific content can be increased by up to approximately 28%. When this reformed fuel is combusted in a furnace, fuel economy is improved, system efficiency is increased, and emissions are reduced. Because both H2O and CO2 can be utilized in the reforming process, it is advantageous for natural gas-fired systems since both of these gases are major products of combustion and are therefore readily available in a preheated state. If steam is available for the process, or if a heat recovery boiler can be installed together with the reformer, then the steam can be used to reform the fuel.
TCR as a process has been investigated for a number of applications (See, e.g., Maruoka N. et al., “Feasibility Study for Recovering Waste Heat in the Steelmaking Industry Using a Chemical Recuperator,” IsIJ International, Vol. 44, 2004, No. 2, pp. 257-262; Yap D. et al., “Natural gas HCCI engine operation with exhaust gas fuel reforming,” International Journal of Hydrogen Energy, 2006, Vol. 31, pp. 587-595; and U.S. Pat. No. 7,207,323.) The results of these investigations showed that a catalyst is required to reform the fuel. Thus, existing recuperative reformers for TCR systems are catalytic.
Most catalysts used in catalytic reformers contain nickel oxide, platinum, or rhenium on a silica, alumina, or a silica-alumina support base, and some contain both platinum and rhenium. Other platinum group elements may also be used. The activity (i.e., effectiveness) of the catalyst in a catalytic reformer is reduced over time during operation by carbon deposition. The activity of the catalyst can be periodically regenerated or restored by in situ high temperature oxidation of the carbon. Typically, catalytic reformers are regenerated about once every 6 to 24 months, and the catalyst normally can be regenerated about 3 or 4 times before it must be returned to the manufacturer for reclamation of the valuable platinum and/or rhenium content.
The use of an expensive catalyst in the reformer increases the reformer capital cost. Further, the necessary periodic regeneration of the catalyst—and the eventual need to replace the catalyst after it is regenerated a few times—also drives up the system cost. As a result, in many applications—especially those with low-temperature waste heat streams, such as automotive applications—the economic benefits of waste heat recovery do not justify the cost of the recovery systems.
Innovative, affordable methods that are highly efficient and suitable for use with corrosive or “dirty” wastes could expand the number of viable applications of waste heat recovery, as well as improve the performance of existing applications.